The invention relates to integrated circuits for use at microwave frequencies, and more particularly to MMIC amplifiers in which the adjustability of critical components is used to simplify the design process and manufacturability of amplifiers required to meet performance specifications. 2. Prior Art
The design and manufacture of an electronic amplifier has always been a matter of substantial complexity. That complexity has increased with the advent of integrated circuits operating at microwave frequencies.
The conventional design procedure for electronic equipment, operating at lower than microwave frequencies, for instance an amplifier, involves compartmenting the amplifier into a succession of stages each comprised of active elements and passive elements. The circuit elements of the amplifier circuit will then receive design values. Resistors will be given resistance values in ohms; capacitors, capacitance values in farads; inductors, inductance values in henrys (the active elements are similarily treated), etc. However, in the physical world, all elements, even at lower frequencies, share measurable amounts of all three properties. The secondary properties, which are often not dealt with in the first stage of the paper design, affect performance so that when the components are assembled, a further iteration in the design procedure is required. The iteration in which the realities of the physical design modify the paper design is termed the "bread board" or "brass board" stage.
After the bread board stage, the question of reproducability or manufacturing is raised. At this point it is decided how to specify the components, which components can be treated as fixed, their tolerance and which components may require adjustment in the interests of achieving peak performance.
Most common electronic equipment (radios, TVs, etc.), until the advent of electronic tuning, used tuning in all circuits operating above audio frequencies. Adjustment is ordinarily labor intensive and the reduction of adjustment costs has been the object of much design activity.
The advent of the integrated circuit changed the ground rules, but continued the inherent complexity of the design and fabrication process. At the lower frequencies, electronic equipment has ordinarily been of a hybrid design in which the active components, the resistors, and small capacitors are a part of the integrated circuit, and the components that cannot be fabricated on the substrate or which require tuning adjustments, are fabricated off the chip. The IC thus required the making of the masks, and the actual fabrication of the IC, together with the testing of the completed amplifier, in the bread board stage.
At microwave frequencies, the design and manufacture of the integrated circuit is now further complicated. One cannot "off-board" components without severe performance penalties. One has to fabricate all the active and passive components whether fixed or adjustable on the integrated circuit.
At microwave frequencies, the components are much more variable than on lower frequencies. A length of transmission line, for instance, depending upon frequency, may appear to be a capacitor, or an inductor, or a resistor. Inductors may become capacitors, and capacitors may become inductors. One must model each component of the amplifier in the complex plane in a manner which recognizes this hightened frequency dependence.
The paper design of the MMIC thus requires that the complicated model of each proposed component be entered into the computer before computer simulation is possible. The computer simulation, however, suffers from the inaccuracies of the models of the individual components. In predicting the performance of the aggregate physical realization, the simulation is often far off the mark.
Even after computer simulation, one must test the MMIC paper design in the physical world. This cannot be done without making the masks and making the actual integrated circuit. The procedure is of considerable expense, and every effort is directed to improve the probability that a second design iteration will not be required.
The need has accordingly arisen for MMIC designs that are of greater predictive accuracy when practically realized and in the event of inaccuracy in the practical realization easily adjusted to achieve optimum performance.